Projecting television image



5. 21, 1941. K. MARTIN ETAL PROJECTING TELEVISION IMAGE Filed Dec. 8, 1937 Fig. 2

Fig 4 Fig, 3

Jnventors: Karl Mar-ti 2 w m "UAR mm. 2 m m .w a N mm ]H PATENT- [OFFICE PROJECTING rsmvrsron macs Karl-Martin and Johannes Fiii'gge, Rathenow, and

' Hans Georg Roll, Jerichow-on-the-Elbe, Germany, asaignors to Emil Busch, 'Aktlen-Gesellschaft, Bathenow, Germany Application December 8, 1937, Serial No. 178,794

In Germany December 14, 1936 7' Claims. (01. 178- 15).

Our invention relates to improvements in television and like receivers of the type wherein'the pictures transmitted are reproduced by means of a cathode ray tube having a fluorescent screen, known as "Braun"-tube for instance from United States Patent 2,039,132; and more particularly to the problem of efliciently magnifying those pictures ofsmall size and moderate brightness,-

which are reproduced on the fluorescent screen 1 of the Braun-tube and projecting them on a viewing screen,-the ultimate object being to their whole area including their marginal sections than obtainable .by television picture receiving and projecting means known and' used here- 25 tofore.

Another equally important object of the'invention consists in projecting magnified television pictures sharply defined and free from chromatic aberration-s by improved optical means 30 cooperatively associated with the Braun-tube, which are of relatively simple design, namely composed of a relatively small number of parts, and distinguished by the absence of strongly curved surfaces of extraordinary shape difiicul-t 35 to be correctly ground and rather expensive as to their total cost-s of manufacture.

Other objects of the invention will become incidentally apparent to experts in the optical and television field as the description proceeds.

0 The nature and scope of this invention are briefly outlined in the appended claims and will be more fully-understood from the following specification taken together with the accompanying drawing, wherein 45 Fig. 1 is a diagrammatic cross section verti-' cally taken through a television receiver having picture magnifying and projecting means re-designed according to this invention;

Figs. 2 to diagrammatically show in cross 50 section optically modified magnifying andprojecting means for television and like receivers designed according to this invention. In the course of their experimental and research work carried out with the objects in view 55 stated above the inventors have found, that by using wide aperture lenses as means for mag: nifying and projecting the small pictures, which are reproduced on the fluorescent screen of Braun-tubes and are of moderate luminescence, the resultant pictures projected on a viewing screenare not sharply defined particularly in their marginal sections, are of greatly reduced luminescence, or else they are inadequately magnified for practical purposes, namely cannot be comfortably seen by a number of persons simultaneously.

It has already been proposed to employ as means for magnifying and projecting television pictures a concave mirror having an aperture, in which the Braun-tube is inserted, and facing both the fluorescent screen of the Braun-tube and the viewing screen as seen in British Patent 452,148. 1

We have not been able to ascertain whether this proposition has been successfully carried out in practice, particularly since the light rays issuing from the fluorescent screen would have to pass the transparent walls of the Braun-tube in oblique direction and would be refracted therein,-resulting in distortion of the pictures to be magnified.

In contradistinction to television receivers of the aforedescribed types the invention employs as magnifying and projecting means a concavel-ens-mirror facing the screen of the Braun-tube I and being spaced therefrom, andan auxiliary lens arranged therebetween,concave lens-mirror in the sense of this invention meaning a concaveconvex' lens from the convex face of which the light rays-refracted in the lens, are reflected. 5

The television receiver re -designed according to this invention and diagrammatically shown in its simplest form in Fig. 1 comprises:

(1) A casing 6 having a viewing screen indicated at B and a reflector Ill cooperatively asso- 40 dated therewith, I

(2) A Braun-tube a arranged in said casing the screen b of which facing the bottom of the casing and being spaced therefrom, and

(3) Picture magnifying and projecting means cooperatively associated with said Braun-tube and the reflector l0 and comprising a concave cording to this invention may have a converging or diverging efiect and should be sodesigned and positionedas to improve the marginal definition 5s oi the pictures projected and eliminate chromatic aberrations. but not to impair the spherical correction characteristic of the lens-mirror 1n proper, the named lens may have also asp-herloal sur-- faces.

Instead of a single auxiliary lens a system of lenses may be used to advantage.

Good results namely amply magnified pictures free from marginal distortions were obtained with a television receiver diagrammatically shown by way of an example in Fig. 2: The Braun-tube a2 of which had a substantially flat screen b2, closely adjacent to which a piano-convex lens oil was arranged for cooperation with the lens-mirror m2; the whole equipment presented the following optical and structural characteristics:

Relative aperture=1:1.4

Equally good results, namely amply magnified pictures free from marginal mstortions and chromatic aberrations and being still brighter in their luminescence were obtained with a television receiver shown by way of another example in Fig. 3: The Braun-tube 13 of which had a slightly vaulted screen b3, and the auxiliary lens system was composed of two lenses spaced from each other, namely a converging meniscus c3, and an achromatic collective lens consisting of two parts 04 and 05, made of different kinds of glass and cemented together; in this case the lensmirror 1123 should be curved considerably sharper than specified with reference to the lens-mirror m2 so as to better comply with the sine condition and to be free from astigmatism, the radius 16 of its refracting face preferably not exceeding three fourths of the focal length of the mirror.

In order to obtain the best proportions, namely lenses not excessively large in diameter and in turn not to impair the luminescence of the projected pictures, the distance of the auxiliary lens system from the lens-mirror m3 should exceed one third of the focal length of the latter, while the distance of the meniscus 03 from the achromatic lens c4, 05 should exceed one fifth of the total or resultant focal length of the whole op tical system.

The longitudinal chromatic aberration of the complete optical equipment can be eliminated, if at least one of the convex lenses of the auxiliary lens system is made of glass with a comparatively low v-value, for instance a flint glass, the v-llldBX of which, indicating its dispersion power relatively to the power of refraction, ranging below 44- as seen at rd and TH on the following table, as against conventional collective lenses of optical systems, the u-index of which generally exceeeding 50.

The optical equipment of television receivers for magnifying and projecting the pictures re-.

produced on the screen 123 of the Brawn-tube shown in Fig. 3 have the following optical and structural characteristics:

Relative aperture 1:0.8

Thickncsses 332: in and distances Kinds of glass in mm.

n=+238 Lens-mirror r;=+48 Meniscus To=+43 Acliroinntic lcns As seen in the drawing and from the above tables, wherein the radii of curvature of lensmirrors designed according to this invention are given, the front face of the lens mirror opposite the viewing end of the Braun tube is in all cases more sharply curved than its rear reflecting face, as will be seen from the fact that the marginal portions of the lens mirror are thicker than the central portions.

Best results were obtained with optical systems of the design described, wherein the focal length of the lens-mirror exceeded 1.5 times the composite focal length of the whole system.

Various other changes and modifications may be conveniently made in the optical characteristics and structural details of television picture magnifying and projecting means of the improved design described and in the assemblage and cooperation of their component parts, without substantially departing from the spirit and the salient ideas of this invention.

. For instance under specific conditions a. diverging meniscus 06 may be used to advantage as auxiliary or correction lens as seen in Fig. 4, and the lens-mirror shown therein may be conveniently made of a convergent lens mi and a divergent lens m5 of different kinds of glass cemented together and so designed as to eliminate chromatic aberrations and to still better comply with the sine condition than possible with lensmirror having only one single lens as retracting element.

On comparing the rate of magnification of television receivers equipped With a lens-mirror and an auxiliary lens according to this invention and enclosed in a box or casing 6 (Fig. 1) of a given height, say two meters, with that of known television receivers having a casing of equal height and having only lenses as picture magnifying and projecting elements, it will be found that the lens-mirror system described allows of a relatively higher magnification, namely because the total length of the path of travel of the light rays from the object to be magnified to the viewing screen includes twice the distance of the fluorescent screen of the Braun-tube from the lens-- mirror which means, that the overall height of television receivers improved according to this invention can be substantially reduced with respect to a given rate of magnification; this is an additional valuable feature and of special advantage for television receivers to be equipped with picture magnifying and projecting means according to this invention and to be used at home.

What we claim is:

1. In combination, television receiving apparatus wherein the picture signals are transmitted to electrodes enclosed in the receiving end of a Braun cathode-ray tube and are reproduced on a fluorescent screen at the viewing end of said tube, a concave lens-mirror arranged opposite the viewing end of said Braun tube coaxially and in spaced relation thereto for magnifying and projecting the reproduced pictures, the front face of said lens-mirror facing the fluorescent screen, being more sharply curved than the reflectin rear face, an auxiliary picture correcting lens arranged between said fluorescent screen and the lens mirror for cooperation with the latter, and means spaced from the receiving end of the Braun tube for receiving the projected pictures.

2. In combination, television receiving apparatus wherein the picture signals are transmitted to electrodes enclosed in the receiving end of a Braun cathode-ray tube and are reproduced on a fluorescent screen at the viewing end of said tube, a concave lens-mirror arranged opposite the viewing end of said Braun tube coaxially and in spaced relation thereto for magnifying and projecting the reproduced pictures, the front face of said lens-mirror facing the fluorescent screen, being more sharply curved than the reflecti rear face, an auxiliary picture correcting lens arranged between said fluorescent screen and the lens mirror for cooperation with the latter, and means spaced from the receiving end of the Braun tube for receiving the projected pictures,

the radius of curvature of the front face of the lens-mirror not exceeding three fourths of the focal length of the lens-mirror, and the distance of the latter from the auxiliary lens exceeding one third of the focal length of the lens-mirror.

3. In combination, television receiving apparatus wherein the picture signals are transmitted to electrodes enclosed in the receiving end of a Braun cathode-ray tube and are reproduced on a fluorescent screen at the viewing end of said tube, a concave lens-mirror arranged opposite the viewing end of said Braun tube coaxially and in spaced relation thereto for magnifying and projecting the reproduced pictures, the front face of to electrodes enclosed in the receiving end of a Braun cathode-ray tube'and are reproduced on a fluorescent screen at the viewing end of said tube, a concave lens-mirror arranged opposite the viewing end of said Braun tube coaxially and in spaced relation thereto for magnifying and projecting the reproduced pictures, the front face of said lens-mirror facing the fluorescent screen, being more sharply curved than the reflecting rear face, an auxiliary picture correcting lens arranged between said fluorescent screen and the lens mirror for cooperation with the latter, and

means spaced from the receiving end of the Braun tube for receiving the projected pictures, said auxiliary lens comprising two collective elements spaced from each other, at least one of said collective elements consisting of glass the v-index of which ranges below 44.

5. 'In combination, television receiving apparatus wherein the picture signals are transmitted to electrodes enclosed in the receiving end of a Braun cathode-ray tube and are reproduced on a fluorescent screen at the'viewing end of said tube, a concave lens-mirror arranged opposite the viewing end of said, Braun tube coaxially and in spaced relation thereto for magnifying and projecting the reproduced pictures, the front face of said lens-mirror facing the fluorescent screen, being more sharply curved than the reflecting rear face, an auxiliary picture correcting lens arranged between said fluorescent screen and the lens-mirror for cooperation with the latter, and means spaced from the receiving end of the Braun tube for receiving the projected pictures, said auxiliary lens comprisingtwo collective elements, a primary element and a secondary element spaced from each other by a distance exceeding one fifth of the focal length of the whole optical system, the secondary element comprising two lenses made of glasses having different refractive indices.

30 6. In combination, television receiving apparatus wherein the picture signals are transmitted to electrodes enclosed in the receiving end of a Braun cathode-ray tube and are reproduced on a fluorescent screen at'the viewing end of said 35 tube, a concave lens-mirror arranged opposite the viewing end of said Braun tube coaxially and in spaced relation thereto for magnifying and projecting the reproduced pictures, the front face of said lens-mirror facing the fluorescent screen, being more sharply curved than the reflecting rear face, an auxiliary picture correcting lens arranged between said fluorescent screen and the lens-mirror for cooperation with the latter, means spaced from the receiving end of the Braun tube for receiving the projected pictures, and a casing of relatively small cross sectional size. wherein the said magnifying picture correcting and projecting elements are arranged in series above each other.

7. In combination, television receiving apparatus wherein the picture signals are transmitted to electrodes enclosed in the receivingend of a Braun cathode-ray tube and are reproduced on a fluorescent screen at the viewing end of said tube, a concave lens-mirror arranged opposite the viewing end of said Braun tube coaxially and in spaced relation thereto for magnifying and projecting the reproduced pictures, the front face of said lens-mirror facing the fluorescentscreen, 

